1. Field of the Invention
The present invention relates to a method for operating a machine tool which comprises a spindle rotatable about a spindle axis; a spindle gripper, carried by the spindle, for gripping workpieces; and a cradle apparatus into which workpieces to be machined can be clamped and can be pivoted back and forth about a pivot axis of the cradle apparatus extending approximately perpendicular to the spindle axis.
A machine tool of this kind is presently being designed in the facilities of the assignee, which are not accessible to the public.
The purpose of the cradle or pivot apparatus is to clamp workpieces in such a way that they can be machined, not only in a horizontal position but also in two vertical positions each pivoted through approximately 90 degrees, by tools clamped in the spindle. The cradle apparatus thus offers, so to speak, an additional machine axis.
With the machine tool described to this point, it is possible to machine workpieces in one clamping not only on its top surface but also on the two side surfaces. The workpieces can, in this context, be held in three positions each pivoted 90 degrees with respect to one another, or also in intermediate positions. Not accessible for machining are the end surfaces of the workpieces, on which clamping jaws of a clamping apparatus arranged on the cradle apparatus engage; and the underside of the workpieces, remote from the top surface, which faces toward the clamping apparatus.
2. Related Prior Art
A machine tool known from DE 36 20 343 C2 is moreover equipped with a spindle gripper which is used to transport workpieces on the workpiece table between a pallet with workpieces held in reserve and a clamping apparatus. In this context, the spindle gripper is inserted into and removed from the spindle in the manner of, for example, an ordinary tool.
It is also known from DE 36 20 343 C2 to configure the spindle gripper rotatably, by equipping it with, for example, a rotary drive actuated by external force. This rotary drive can be activated, for example, by fluids or electrical power; the fluid and/or power can be supplied through the spindle itself. Rotatable spindle grippers of this kind are used for positionally correct adjustment of workpieces to tools installed immovably on the workpiece table, or for positionally correct placement of the machined workpieces onto a second pallet.
In the case of the aforementioned machine tool, when the underside and/or the end surfaces of the workpieces also need to be machined, the workpieces must be reclamped for that purpose. This is done either manually or with an additional handling device, for example in the form of a robot arm.
In the case of manual reclamping, it is disadvantageous that the required workpiece reclamping time is relatively long. A further disadvantage consists in the fact that manual reclamping entails a certain inaccuracy, so that positionally correct association between holes made before and after reclamping often cannot be maintained.
The use of an additional handling device leads to an overall increase in machine costs, and has the further disadvantage that a portion of the surface of the workpiece table is occupied by said handling device and is thus no longer available for other tasks. A further disadvantage of the handling device is that collisions between the handling device and the spindle must be avoided, which requires a considerable expansion of the usual machine control system.